This sophomore effort by Kasi Lemmons, who made her directorial debut with 1997’s subtle, evocative “Eve’s Bayou,” isn’t really a good movie, but it’s an oddly engrossing one. A strange amalgam of character study and murder mystery, “The Caveman’s Valentine” boasts an intense lead performance by Samuel L. Jackson and has been fashioned by Lemmons with great visual panache, but while the picture beguiles the eye, it’s much less successful in satisfying the brain. Simply put, even the combination of a fine actor and a skillful helmer can’t sufficiently compensate for the inadequacies of the script that George Dawes Green has fashioned from his own novel.

The plot centers on a Romulus Ledbetter (Jackson), a once brilliant musician whose paranoid delusions have left him a homeless inhabitant of a cave in a New York City Park. Romulus, we soon learn, adamantly refuses any official assistance, convinced that society is governed by a malevolent entity named Stuyvesant who, in the imaginings of his tortured mind, rules from atop the Chrysler Building; Ledbetter wanders about the streets shouting down anyone who scrutinizes him, remembering happier times with his daughter Lulu, now a policewoman (Aunjanue Ellis), and occasionally being visited–and taunted–by the shade of his estranged wife Sheila (Tamara Tunie). Romulus’ routine suddenly changes when he finds a young man frozen to death, propped up in a tree near his cave; at the urging of a street pal (Rodney Eastman), who knew the fellow, he undertakes to investigate the death. This effort not only reunites him with his exasperated daughter, but also leads him into the circle of celebrated photographer David Leppenraub (Colm Feore), who had utilized the dead boy as a model. Romulus becomes fixated on solving the mystery of the youth’s demise as a means of defeating the plans of his nemesis Stuyvesant, who–in his derangement–he identifies as the party ultimately responsible. His obsessive investigation comes to involve Leppenraub’s sister (Ann Magnuson), a young acolyte of the artist (Jay Rodan) and a breezy bankruptcy lawyer (Anthony Michael Hall) who helps make the scruffy fellow presentable in upper-crust society again, thus allowing him to carry out his inquiries.

What works in “The Caveman’s Valentine” is mostly Jackson’s riveting performance; he takes considerable risks to draw a credible portrait of a man whose remarkable abilities have been wasted by mental illness. Jackson rants, he shivers, he snarls–but he manages occasional moments of convincing lucidity and peace. It’s a tour de force of a turn which, in a better context, would be overwhelming. Unfortunately, the mystery element of the narrative lets the actor down. The circumstances of the young man’s death are contrived to begin with, but as Romulus delves more deeply into his demise, things get ever more ridiculous and absurd; and the revelatory denouement–the sort of “gotcha” moment that might be found in a bad Agatha Christie novel–is simply preposterous. Lemmons tries to divert the viewer’s attention from the utter implausibility of it all by focusing on Jackson’s haunted face and choreographing lots of surrealistic inserts reflective of Romulus’ crazed imaginings and recollections, but their repetitiveness eventually grows tiresome, however effective they might be on first appearance. She does, however, secure calm, restrained performances from the supporting cast, with Feore and Magnuson registering nicely and even Hall (absent from serious features for years, but all too often present in straight-to-video junk) coming off well. Ultimately, however, neither the thespian talent on display nor Lemmons’ visually striking approach can make up for a story which sometimes seem as wrongheaded as its protagonist. “The Caveman’s Valentine” will hold your attention, but you’re likely to keep watching more out of morbid curiosity than any sense of respect or pleasure. Lemmons, Jackson and the result of the cast really deserve better than this; and so do we.